The present invention relates to a hydraulic brake system with slip control, in particular for motor vehicles, in which at least one master cylinder piston is provided for the operation of a vehicular brake, in which hydraulic medium tapped from the wheel brake during slip control is supplied to the wheel brake from an auxiliary pressure source via a master cylinder gasket acting as check valve, and in which the operating stroke of the master cylinder piston is limited by pressurization of a positioning piston through which a master cylinder piston rod is passed in a sealed manner and which is sealingly guided and axially displaceably in a housing.
In a brake system according to a prior art arrangement a tandem master cylinder is used which is operable by a hydraulic brake booster. The hydraulic brake booster has a booster piston accommodating a brake valve which permits establishing a pressure in the pressure chamber of the hydraulic booster which is proportional to the operating force. A first brake circuit is connected to the pressure chamber of the hydraulic brake booster leading to the wheel brakes of the rear axle of a motor vehicle.
Upon displacement of the booster piston there will be a pressurization of the working chambers of the tandem master cylinder. Each of the working chambers of the tandem master cylinder has communication with a different wheel brake preferably situated at the front axle of the vehicle. Thus, within a three-circuit brake system, each of the front wheels are pressurized separately in a static manner while the actuation of the rear wheel brakes will be dynamic.
Associated with the vehicular brakes are electromagnetically operable valves switchable by control electronics so as to ensure that pressure medium will be tapped from one or several wheel brakes in the case of critical lock-up values. The pressure medium tapped from the dynamic brake circuit will be directly replaced from an auxiliary pressure source via the brake valve. Pressure medium tapped from the static circuits will be replaced by the pressurization of the front or end surfaces of the corresponding master cylinder pistons adjacent the brake pedal so that pressure medium may flow via axial channels in the master cylinder piston and a gasket, acting as check valve, into the working chambers of the tandem master cylinder.
In order to safeguard that there will always remain a certain residual volume in the working chambers of the tandem master cylinder, the brake apparatus has a positioning piston whose front or end surface remote from the brake pedal is pressurized by the pressure of the dynamic circuit during slip control. The positioning piston essentially has the construction of a hollow cylinder. The positioning piston is sealed at its outside circumference and guided in a bore of the housing. Within the internal bore of the positioning piston the master cylinder piston rod is axially displaceably guided in a sealed manner. Upon pressurization of the positioning piston, the positioning piston will move in certain operational conditions against an enlarged circumferential end of the master cylinder piston rod. Thus, the positioning piston will keep the master cylinder pistons in a defined position in which there will be available a sufficient volume reserve in the working chambers of the tandem master cylinder. Upon pressurization, under certain conditions, the positioning piston will abruptly abut at the enlarged circumferential end of the master cylinder piston rod which will cause a corresponding jerk at the brake pedal. This jerk will inform the driver of the vehicle of the onset of the control of the brake slip. On the other hand, however, the abrupt return of the brake pedal may also lead to a distraction of the driver's attention to traffic in driving situations which are critical anyway.